Astronomy

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Jesse Emspak is a science writer who lives

and works in New York City.

equation, E=mc^2 .) But at a certain point,
if there’s no mass detected, one has to
wonder if it’s massless — or even exists.
Meanwhile, other scientists have
taken a crack at looking for quantum
effects at large scales using combinations
of black holes and neutron stars. Neutron
stars are the corpses of stars born with
more than about eight times the Sun’s
mass, but less than the 20 solar masses
needed to make black holes. They have
powerful magnetic fields, and some send
focused beams of radio waves in our
direction at regular intervals like a light-
house beacon — pulsars.
John Estes of Long Island University
and his colleagues have proposed using
the precisely timed signals from a pulsar
orbiting a black hole to probe the region
near the event horizon. Since black holes
bend light, the pulsar’s signal would be
delayed by a discrete amount when the

pulsar passes behind the black hole. If

quantum effects are important, then that

delay would change in ways general rela-

tivity cannot predict, and might even

reveal something about how quantum

mechanics works with relativity.

But there’s another reason to do these

kinds of tests. It is far from clear that

gravity — and thus general relativity

— applies the same way at different

scales. Leo Stein of Caltech notes that

the physics one encounters near a super-

massive black hole, like the one at the

center of the Milky Way, might be dif-

ferent from the physics near a stellar-

mass black hole.

LIGO, for example, showed us that

gravitational waves are generated in the

expected way near a stellar-mass black

hole. The steepness of the curvature of

space just outside the event horizon is

smaller for larger black holes, however.

“It’s potentially very different physics,”

Stein says. “When curvatures are large,

you need to use LIGO. When interested

in physics on the millions-of-kilometers

scale, there are things you might be able

to test with the EHT. In the middle is

stuff we have got nailed down, at the

solar system scale.

“Cosmologists are interested in

changing the theory of gravity at very

large distance scales,” he says. “String

theorists and high-energy physicists are

changing the theory of gravity at the

quantum level.”

As with many mysteries, however, we

may have to wait for observations to tell

us one way or the other which theory is

correct. “The puzzle is getting completed

one piece at a time,” says Yunes.